The present invention relates to a combustion controller for lean burn type internal combustion engines. More specifically, the present invention pertains to a combustion controller for lean burn engines having a nitrogen oxide storage-reduction catalyst in their exhaust passages.
Generally, in conventional engines, fuel is injected into an intake port located upstream of a combustion chamber. The injected fuel is mixed with air in the port. In this way, a homogeneous mixture of fuel and air is supplied to the combustion chamber. In this type of engine, a throttle valve adjusts the opening degree of the intake passage. The throttle valve moves by accelerator manipulation. Accordingly, the amount of the mixture supplied to the combustion chamber is adjusted, and this controls the engine output.
However, in such a homogeneous combustion engine, low negative pressure, or great vacuum, is generated in the intake passage when the throttle valve moves. This increases pumping losses and lowers intake efficiency. On the other hand, some engines also use stratified charge combustion. In this technology, fuel is directly supplied to the combustion chamber with the intake passage widely opened, which results in an easy-to-burn mixture in the vicinity of the ignition plug.
In stratified combustion, when the engine load is low, fuel is directly injected into the combustion chamber. More precisely, fuel is injected in the vicinity of the ignition plug. The injected fuel burns with the intake air delivered from a widely open throttle valve. This lowers pumping losses. Further, stratified combustion engines can be operated with a lean air-fuel ratio, improving combustion efficiency.
In lean air-fuel ratio combustion, nitrogen oxide (NOx) is likely to be produced. To purify NOx, a NOx storage-reduction catalyst (NOx absorbent) is used in stratified combustion engines. A main component of the NOx storage-reduction catalyst is zeolite, which carries platinum. The zeolite stores NOx from the exhaust gas during the normal exhaust gas state. On the other hand, when the density of unburned hydrocarbons (HC) in the exhaust is high, the NOx stored in the catalyst is reduced by the HC and emitted as nitrogen gas (N.sub.2).
Japanese Unexamined Publication No. 8-319862 describes an apparatus having a NOx storage-reduction catalyst. In this apparatus, so-called rich-spike control is performed. When the engine is continuously operated with a lean air-fuel ratio, the amount of NOx stored in the NOx catalyst reaches saturation, and extra NOx, not stored in the catalyst, is emitted in the exhaust gas. To prevent this, the air-fuel ratio is controlled to be temporarily rich, or "spiked", during a predetermined time interval. Then, the amount of HC in the exhaust gas increases and the NOx stored in the catalyst is reduced and emitted as nitrogen gas (N.sub.2).
Furthermore, in the apparatus of the Japanese Publication, a fuel injection amount is controlled to prevent a sudden torque change caused by rich-spike control. That is, the fuel injection amount is set such that the output torque during rich-spike control, based on the injection amount, will be equal to the output torque before rich-spike control.
However, engines are subject to many control variables. Under normal operation, various kinds of engine control parameters such as the air intake amount, the exhaust gas recirculation (EGR) amount, the fuel injection amount, the ignition timing are frequently changed when torque fluctuation and knocking are controlled. When these parameters are changed, engine output (torque) is likely to change. In the prior art, when output torque changes based on various engine controls, rich-spike control may wrongly recognize a target torque state and set the fuel injection amount to a wrong value. As a result, a sudden torque change may occur.